Synthesis, Crystal Structure, and Electrical Conductivity of Pd-intercalated NbSe2

HUANG Chong, ZHAO Wei, WANG Dong, BU Kejun, WANG Sishun, HUANG Fuqiang,3

Synthesis, Crystal Structure, and Electrical Conductivity of Pd-intercalated NbSe2

HUANG Chong1,2, ZHAO Wei1, WANG Dong1, BU Kejun1,2, WANG Sishun1, HUANG Fuqiang1,3

(1. State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China; 2. University of Chinese Academy of Sciences, Beijing 100049, China; 3. State Key Laboratory of Rare Earth Materials Chemistry and Applications and Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China)

New intercalated compounds PdNbSe2(=0-0.17) were synthesizedsolid-state reaction. They possess the parent structure of 2H-NbSe2and crystalize in the hexagonal space group ofP63/mmc. The intercalated Pd occupies the octahedral position in the van der Waals gaps of 2H-NbSe2. Unit cell parameterincreases linearly with the Pd content, whileis nearly unchanged. The lattice parameter of Pd0.17NbSe2(==0.34611(2) nm,= 1.27004(11) nm) is identified by single crystal X-ray diffraction. The intercalated Pd stabilizes the crystal structure of NbSe2by connecting the adjacent Nb-Se layers with [PdSe6] octahedra and leads to the enhanced thermostability in air. Temperature dependence of electric resistivity reveals that the residual resistivity ratio of PdNbSe2monotonically decreases with addition of the intercalated Pd content. The decreased superconducting critical temperature of PdNbSe2indicates the suppression effect of Pd intercalation on the superconductivity in the host NbSe2.

PdNbSe2; transition metal dichalcogenide; crystal structure; superconducting

Layered transition metal dichalcogenides (TMDs) with the general chemical formula MX2(M represents the transition metal and X is the chalcogen) have been widely studied due to their unique physicochemical properties and diverse applications[1-5]. The metallic Group ⅤB TMDs (where M = V, Nb and Ta) are prized for their fascinating electronic properties, such as charge density wave (CDW), superconductivity and Mott transition[6-7]. Among them, 2H-NbSe2is featured with a high superconducting critical temperature (C) of ~7.3 K and a quasi-two-dimensional incommensurate charge density wave (ICDW) with aCDWof ~33 K[8]. Because of the weak van der Waals (vdW) forces connected interlayers in the crystal structure, 2H-NbSe2can be intercalated by various guests, including atoms, ions, and molecules[9].

Typically, the incorporation of guest metal atoms into the vdW gaps of TMDs could give rise to the crystallographic transformation and change of electronic structure in the intercalated compounds[10]. Magnetic elements (Fe, Co) inserted into the vdW gaps of NbSe2resulted in the formation of superlattice[11-12]. Alkali metal intercalation was found to remove the CDW instability in NbSe2[13]. Recently, noble metal, such as palladium (Pd), was applied to regulate the electronic structure efficiently for the host TMDs. Our group[14]found that Pd modified the band structure of 2H-TaS2through Pd−S bonding to strengthen the interaction of adjacent Ta-S layers, which led to the enhanced conductivity in Pd0.10TaS2. Pd intercalation was reported to increase the effective electron- phonon coupling in 2H-TaSe2and enhance theCin PdTaSe2[15]. Considering that the crystal structure of 2H-NbSe2is identical to that of 2H-TaX2(X=S, Se), Pd intercalation should be applicable to 2H-NbSe2and tune the physical properties.

In this work, a series of new compounds PdNbSe2(=0~0.17) were synthesized and the crystal structure of Pd0.17NbSe2was determined by single X-ray diffraction method in order to investigate the modification of crystal lattice and electrical conductivity in the Pd intercalated NbSe2.

1 Experimental

1.1 Preparation of PdxNbSe2

PdNbSe2crystals were prepared by solid-state reaction. Pd (99.99%), Nb (99.5%) and Se (99.99%) powders were mixed according to stoichiometric ratio, and ground. Then the mixtures were compacted into a pellet and heated in the evacuated (< 0.133 Pa) silica tube at 1173 K for 48 h. Subsequently, the as-obtained samples were reground, re-pelletized and held at 1173 K for 72 h. Then the samples were cooled down by quenching in water. High quality single crystal of Pd0.17NbSe2was obtained by keeping Pd0.17NbSe2powder with CsI (99.9%) at 1173 K for 1 d and slowly cooling down to 823 K for 3 d.

1.2 Characterization

Single crystal data collections of Pd0.17NbSe2was conducted on a Bruker D8 QUEST diffractometer equipped with Mo Kα radiation at room temperature. The crystal structure determination and refinement were performed with the APEX3 program. The crystal structure of Pd0.17NbSe2was drawn by using the VESTA program[16]. The morphology and the composition of the Pd0.17NbSe2were investigated by a scanning electron microscope (SEM, JSM6510) coupled with energy dispersive X-ray spectroscopy (EDXS, Oxford Instruments). The micro-structure of Pd0.17NbSe2was uncovered by a high-resolution transmission electron microscope (HRTEM, JEM-2100F) and the selected area electron diffraction (SAED). The valence analysis of the Pd0.17NbSe2was obtained from X-ray photoelectron spectroscope (XPS) carried out on the RBD upgraded PHI-5000C ESCA system (PerkinElmer) with Mg Kα radiation (=51253.6 eV). The binding energy in XPS analysis was corrected by referencing C 1s peak at 284.6 eV. Powder X-ray diffraction (PXRD) data of these PdNbSe2samples were collected by using a Bruker D8QUEST diffractometer equipped with Cu Kαradiation (=0.15405 nm). Thermogravimetric analysis (TG) and differential thermal analysis (DTA) were carried out on a NETZSCH STA449C thermal analyzer for investigating the thermal stability of Pd0.17NbSe2and NbSe2in air. Resistivity of the as-obtained PdNbSe2at different temperatures was executed on a Physical Properties Measurement System (PPMS, Quantum Design). A four-probe method was adopted for measurements of the resistance. More specifically, the powders were pressed into a disk. Silver paste and copper wire acted as the contact electrode and conduct wire, respectively. Normalized resistivity (/300 K)temperature curves were obtained via dividing the measured resistivity () by the resistivity value (300 K) at room temperature.

2 Results and discussion

The crystal structure of Pd0.17NbSe2identified by single crystal X-ray diffraction method is shown in Fig. 1(a-b), where the gray, blue, orange spheres represent Pd, Nb, and Se atoms, respectively. The crystal data and structure refinement of Pd0.17NbSe2are given in Table 1. The fractional atomic coordinates and equivalent isotropic displacement parameters are summarized in Table S1. The atomic displacement parameters and the geometric pa rameters are shown in Table S2–S3. The space group of Pd0.17NbSe2is determined to be P63/mmc with lattice parameters of=0.34611(2) nm,=1.27004(11) nm. Pd0.17NbSe2contains one independent Nb site (2), one independent Se site (4) and one independent Pd site (2). Pd0.17NbSe2consists of Nb-Se layer and Pd-Se layer, which are stacked alternately alongaxis. Each Nb atom is coordinated by 6 Se atoms which formed a [NbSe6] triangular prism (Fig. 1(c)). The length of Nb−Se bond in [NbSe6] triangular prism is 0.26006(4) nm which is comparable to 0.25941(5) nm in NbSe2[17]. These [NbSe6] triangular prisms are connected by edge-sharing to form the Nb-Se layer.

Fig. 1 Crystal structure of Pd0.17NbSe2 along (a) the bc-plane and (b) the ab-plane, (c) [NbSe6] triangular prism in Pd0.17NbSe2, (d) [PdSe6] octahedron in Pd0.17NbSe2

Table 1 Crystal data and structure refinement of Pd0.17NbSe2

1=Σ||o|−|c||/Σ|o|,2=[Σ(o2−c2)2/Σ(o2)2]1/2,=1/ [2(o2)+()2+], whereois the observed structure factor,cis thecalculated structure factor,is the standard deviation ofc2, and=(o2+2c2)/3.=[Σ(o2−c2)2/(−)]1/2, whereis the number ofreflections andis the total number of parameters refined.

Each Pd atom is coordinated by 6 Se atoms to form [PdSe6] octahedron (Fig. 1(d)). The average length of Pd−Se bond in [PdSe6] octahedron is 0.25051(4) nm which is comparable to 0.248602(0) nm of PdSe2[18]. These [PdSe6] octahedra partially fill in the vdW gaps of NbSe2, where the occupation of Pd sites is 17%.

The Pd0.17NbSe2plate with the size about 5 μm was observed by SEM (Fig. 2(a)). The Pd atoms are in a homogenous dispersion in Pd0.17NbSe2, which is confirmed by the elemental mapping analysis of Pd0.17NbSe2. HRTEM image of Pd0.17NbSe2(Fig. 2(b)) reveals that the lattice fringes with a spacing of 0.301 nm are assigned to (101) plane and (11¯1) plane between which the angle is 60°. This result is also verified by the corresponding SAED.

XPS data was obtained to confirm the valence state variation of the elements in Pd0.17NbSe2. As displayed in Fig. 3(a), the Pd 3d region is the only difference between Pd0.17NbSe2and NbSe2. The Pd 3d region of Pd0.17NbSe2shows two peaks, which locate at the binding energy of 341.95 eV (3d3/2) and 336.70 eV (3d5/2) (Fig. 3(b)). The valance state of Pd in Pd0.17NbSe2is identified as +2 according to these two peaks[14]. There are two peaks locating at 55.27 (Se 3d3/2) and 54.50 eV (Se 3d5/2) in the Se 3d region of Pd0.17NbSe2, similar to those in the Se 3d region of NbSe2(Se 3d3/2at 55.25 eV and Se 3d5/2at 54.49 eV) (Fig. 3(c)). Therefore, the valance state of Se in Pd0.17NbSe2is considered as –2. The Nb 3d region shows a mixture of oxidation states because of the slightly oxidation of the samples (Fig. 3(d))[19]. The peaks locating at 206.93 and 204.20 eV are attributed to the Nb−Se bonding in Pd0.17NbSe2. In comparison with these two peaks in pristine NbSe2(207.01 and 204.25 eV), there is a slight redshift in Pd0.17NbSe2, implying the partial reduction of Nb as a result of Pd intercalation[14].

Fig. 2 (a) SEM images of Pd0.17NbSe2 and the corresponding elemental mapping analysis, and (b) HRTEM image of Pd0.17NbSe2 along [101¯] zone axis with inset showing the corresponding SAED pattern

Fig. 3 XPS results of Pd0.17NbSe2 and NbSe2

(a) Survey spectra, (b) Pd 3d spectrum of Pd0.17NbSe2, (c) Se 3d spectrum, and (d) Nb 3d spectrum

The intercalated amounts of Pd in NbSe2could be variable, resulting in the formation of a series of PdNbSe2. The powder XRD patterns of PdNbSe2are displayed in Fig. 4(a), with the pristine NbSe2as reference. The peaks of Pd0.17NbSe2are well matched to the simulated one obtained from single crystal data, which suggests a high degree of phase purity. The NbSe2still maintains its space group (P63/mmc) after Pd intercalation. The (004) peak gradually shifts to a lower angle compared with 2H-NbSe2. Furthermore, the lattice parameterundergoes a negligible change. In a sharp contrast, lattice parameterincreases remarkably because of Pd intercalation enlarging the interlayer space of NbSe2(Fig. 4(b)).

The influence of Pd intercalation on the thermostability of the samples was investigated. As clearly seen in Fig. 5(a), the weight of NbSe2begins to increase slightly at 559 K due to the formation of Nb2Se4O13[20]. Subsequently, TG curve of NbSe2suffers a dramatic decrease because of the complete oxidation of NbSe2to Nb2O5. However, the process of mass increase could not be found in Pd0.17NbSe2, suggesting that the intercalated Pd enhances the thermostability of NbSe2with a higher oxidizing temperature. According to DTA curves (Fig. 5(b)), the oxidizing temperature of Pd0.17NbSe2is 608 K, higher than NbSe2(544 K). The enhanced thermostability in air could stem from the intercalated Pd which stabilizes the crystal structure of NbSe2by connecting the adjacent Nb-Se layers[11,21-22].

The electrical conductivity of PdNbSe2was measured by PPMS. The resistivity of PdNbSe2increases with the rising temperature (Fig. S1) exhibiting metallic behavior. Moreover, the residual resistivity ratio () [(resistivity at 300 K)/(resistivity just aboveC)] for the Pd0.17NbSe2is ~1.09, extremely lower than NbSe2(~7.67) (Fig. 6(a)). The poorin Pd0.17NbSe2indicates that the intercalated Pd may be an electronically disruptive dopant in NbSe2, which is similar to the copper (Cu) in CuNbSe2and the gallium (Ga) in GaNbSe2[8,23]. All of the PdNbSe2samples exhibit a sharp decrease at low temperature region from 8 K to 2K, indicating that the superconductivity occurs in these samples. Fig. 6(b) shows that theCdecreases with a higher intercalated amount of Pd (7.4 K for NbSe2and 2.7 K for Pd0.17NbSe2). Eventually, the zero resistivity cannot be observed at 2 K in Pd0.17NbSe2. Therefore, it declares that the intercalated Pd has a negative effect on the superconductivity in NbSe2. Similar phenomena are also found in CuNbSe2, GaNbSe2, FeNbSe2and AlNbSe2[8,23-24]. The reason for this might be that Pd intercalation disrupts the coherence of the CDW, and suppresses the pairing channel which contributes to the higherCin NbSe2[8].

Fig. 4 (a) Powder XRD patterns of PdxNbSe2 (x=0, 0.05, 0.10, 0.15, 0.17), (b) composition dependence of the lattice parameters a and c for PdxNbSe2 (0≤x≤0.17)

Fig. 5 (a) TG and (b) DTA curves of Pd0.17NbSe2 (blue) and NbSe2 (red)

Fig. 6 (a) Temperature dependence of the RRR (ρ/ρ300 K) for PdxNbSe2 (0≤x≤0.17) with inset showing enlarged temperature regionof the superconducting transition, (b) composition dependence of TC

3 Conclusions

In summary, we introduced noble metal Pd into the vdW gaps of NbSe2, and synthesized a series of new intercalated compounds PdNbSe2. The Pd0.17NbSe2crystalizes in hexagonal structure with cell parameter= 0.34611(2) nm,=1.27004(11) nm. The intercalated Pd stabilizes the crystal structure of NbSe2by connecting the adjacent Nb-Se layers with [PdSe6] octahedra leading to the enhanced thermostability in air. PdNbSe2remains the metallic character, which is verified by the resistivity measurements. In addition, the incorporation of Pd decreases theCof NbSe2, implying that Pd is negative for the superconductivity in NbSe2.

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Pd插层NbSe2化合物的制备、晶体结构和电学性质研究

黄冲1,2, 赵伟1, 王东1, 卜克军1,2, 王思顺1, 黄富强1,3

(1. 中国科学院 上海硅酸盐研究所, 高性能陶瓷和超微结构国家重点实验室, 上海 200050; 2. 中国科学院大学, 北京 100049; 3. 北京大学 化学与分子工程学院, 北京分子科学国家实验室, 稀土材料化学及应用国家重点实验室, 北京 100871)

通过固相反应法合成一系列插层化合物PdNbSe2(=0~0.17)。它们与2H-NbSe2相同, 属于六方晶格, 空间群为P63/mmc。Pd占据NbSe2层间的八面体空位。随着Pd含量的增加, 晶格常数线性增大, 而几乎不变。X射线单晶衍射结果表明, Pd0.17NbSe2的晶格常数为==0.34611(2) nm,=1.27004(11) nm。每个Pd原子与六个Se原子键合形成[PdSe6]八面体来连接相邻的Nb-Se层, 使晶体结构变得更加稳定, 从而提高化合物的热稳定性。电学测试表明, 随着Pd含量的增加, PdNbSe2的剩余电阻比减小。此外, 超导转变温度也随着Pd含量的增加而下降, 说明Pd的引入不利于NbSe2的超导态。

PdNbSe2; 过渡金属硫族化合物; 晶体结构; 超导

O782

A

2019-03-26;

2019-04-30

National Key Research and Development Program (2016YFB0901600); Science and Technology Commission of Shanghai (16JC1401700, 16ZR1440500); National Natural Science Foundation of China (Y93GJ11101); The Key Research Program of Chinese Academy of Sciences (QYZDJ-SSW-JSC013, KGZD-EW-T06); CAS Center for Excellence in Superconducting Electronics, and Youth Innovation Promotion Association CAS

HUANG Chong (1994–), male, Master candidate. E-mail: huangchong@student.sic.ac.cn

黄冲(1994–), 男, 硕士研究生. E-mail: huangchong@student.sic.ac.cn

HUANG Fuqiang, professor. E-mail: huangfq@mail.sic.ac.cn; ZHAO Wei, associate professor. E-mail: zhaowei220@mail.sic.ac.cn

黄富强, 研究员. E-mail: huangfq@mail.sic.ac.cn; 赵伟, 副研究员. E-mail: zhaowei220@mail.sic.ac.cn

1000-324X(2020)04-0505-06

10.15541/jim20190125